1. Field of the Invention
The present invention relates to an electrically tunable wavelength-selective filter whose resonance wavelength is variable so that it can select a desired optical signal of an intended wavelength from wavelength division multiplexed optical signals transmitted through an optical fiber.
2. Description of the Prior Art
Optical fiber communications have increased at a rapid rate recently, because of their tremendous information carrying capacity. The current optical communications, however, transmit only a coded pulse stream, ignoring wavelength information. Transmission of many optical pulse streams of diverse wavelengths might further increase information carrying capacity. This technique is termed wavelength division multiplexing (WDM), and has been intensively studied. In wavelength division multiplexing, a tunable wavelength-selective filter is needed that can select an intended optical signal from a great number of optical signals of different wavelengths. In particular, a filter is required having a narrow bandwidth, a wide tunable range, and a low loss.
Conventional filters of this kind include a grating monochromator whose grating is controlled by a motor, an etalon whose resonator length is controlled by a piezoelectric cell, a semiconductor optical waveguide tunable wavelength-selective filter including a Bragg reflector, and a planar light wave Mach-Zehnder interferometer formed on an Si substrate. Each of them, however, has its own disadvantages: The grating monochromator and the etalon are bulky because they are mechanically controlled; the semiconductor optical waveguide filter has only a narrow tunable range and the Mach-Zehnder interferometer must be connected in a number of stages in cascade, and further requires a complicated control system.
To eliminate the disadvantages of the mechanical filters or the semiconductor optical waveguide filter, we proposed a tunable liquid crystal wavelength-selective filter. It includes a liquid crystal contained in a Fabry-Perot interferometer, and its optical length can be varied by applying a voltage (Japanese Patent Application No. 2-71901, 1990).
A tunable liquid crystal wavelength-selective filter is characterized by such features as small size, low driving voltage, and low cost.
FIG. 1 is cross-sectional view illustrating an arrangement of a conventional tunable liquid crystal wavelength-selective filter. It comprises a liquid crystal 1 sandwiched between alignment layers 3A and 3B, dielectric mirrors 4A and 4B, transparent electrodes 5A and 5B, glass substrates 8A and 8B, and antireflection (AR) coatings 9A and 9B. Its cavity gap, that is, the distance between the two dielectric mirrors 4A and 4B is on the order of a few micrometers to an order around ten micrometers. The liquid crystal 1 is a nematic liquid crystal, and its molecules are aligned parallel to the surface (homogeneous ordering).
Typical characteristics of the conventional tunable liquid crystal wavelength-selective filter are as follows: The bandwidth is approximately 0.3-0.6 nm; the loss is 2-3 dB: the tunable range is about 50-100 nm; and the finesse is 150-250. In an etalon filter, a range five times the bandwidth gives an extinction ratio of about 20 dB, and hence, wavelength spacing of 2 nm enables the filter to be applied to wavelength division multiplexing with 50 divisions. Frequency division multiplexing (FDM), however, requires a bandwidth equal to or less than 0.1 nm in practice. The bandwidth can be narrowed by increasing the cavity length of an etalon filter. A cavity gap of 70 .mu.m, for example, gave a bandwidth of 0.1 nm and a tunable range of 10 nm, although it cause the problem that its loss increase to 10 dB. In addition, the 70 .mu.m cavity gap remarkably delay the response time, to the order of several tens of seconds.
Furthermore, the tunable liquid crystal wavelength selective filter has another disadvantage in that it exhibits polarization dependence. In other words, although it operates as a tunable wavelength-selective filter for light whose polarization direction is parallel to the liquid crystal molecules, it cannot operate for light whose polarization direction is perpendicular to the liquid crystal molecules.
Table 1 shows the characteristics of the above-mentioned tunable wavelength-selective filters for purposes of comparison. All the filters have a limited number of selective channels, from several tens to one hundred, and hence, a narrower bandwidth and a wider tunable range are required.
TABLE 1 ______________________________________ CHARACTERISTICS OF TUNABLE WAVELENGTH-SELECTIVE FILTER Tunable Bandwidth Range LOSS Selective Filter (GHz) (nm) (dB) Number ______________________________________ 1 &lt;several tens &gt;100 1-2 100 2 10 3 10 10 3 5 10 3-5 128 4 38 30 3 Several tens 5 60 140 2 50 ______________________________________ NOTES: 1: mechanical grating monochromator 2: semiconductor filter 3: waveguide MachZehnder interferometer 4: mechanical fiber FabryPerot Interferometer 5: liquid crystal filter 125 GHz corresponds to 1 nm.
Incidentally, the tunable liquid crystal wavelength-selective filter is described in the following articles:
(1) Masashi HASHIMOTO "An Optical Resonator Type Wavelength Selector Using Liquid Crystal (2)", 1986, Japan.
(2) Stephen R. Mallinson "Wavelength-selective filters for single-mode fiber WDM systems using Fabry-Perot interferometers", APPLIED OPTICS, Vol. 26, No. 3, Feb. 1, 1987.
(3) M. W. Maeda, et al. "Novel Electrically Tunable Fiber Based on a Liquid-Crystal Fabry-Perot Etalon for High-Density WDM Systems", ECOC '90-145.
(4) M. W. Maeda, et al. "Electronically Tunable Liquid-crystal-Etalon Filter for High-Density WDM Systems", IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 2, NO. 11, NOVEMBER 1990.
On the other hand, wavelength division multiplexing requires a photodetector that has a tunable wavelength-selective function so that an intended optical signal is selected from a number of optical signals of different wavelengths.
FIGS. 2 and 3 illustrate conventional photodetectors with a tunable wavelength-selective filter.
In FIG. 2, an intended wavelength is variably selected by adjusting the angle of a grating 21. Reference numeral 22 designates a lens. To improve the resolution, it is necessary to elongate the distance between an incident fiber 24 and a photodetector 23, which makes the arrangement large. In addition, it is fragile against mechanical shocks.
To eliminate these disadvantages, a photodetector as shown in FIG. 3 is proposed. It comprises a tunable liquid crystal wavelength-selective filter 35 having liquid crystal retained in a Fabry-Perot interferometer. Reference numerals 38 and 29 denote birefringent prisms and .lambda./2 plates, respectively. This photodetector has advantages in that it is small in size, uses a low driving voltage, and is low cost. In addition, since it is solid state, it is strong against mechanical shock. However, it requires a considerable effort for fiber coupling alignment because single-mode fibers 34 are connected to both ends. Further, it costs much because birefringent prisms 38, or polarization beam splitters as their alternatives, are needed at the input side and the output side.